Composite materials

ABSTRACT

Light weight composites with high flexural strength comprise epoxy foam sandwiched between two layers of facing material have high strength and low weight and can be used to replace steel structures. The facing layer may be fibrous material especially glass or carbon fibres, the facing material is preferably embedded into the epoxy matrix. Alternatively they may be matching box structures or concentric metal tubes. The sandwich structures may be prepared by laying up the fibre; coating and/or impregnating the layer with epoxy resin, laying a layer of heat activatable foamable epoxy material, providing a further layer of the fibrous material optionally coated and/or impregnated with epoxy resin on the foamable material and heating to foam and cure the epoxy materials. Alternatively they may be formed by extrusion of the foamable material between the surface layers.

TECHNICAL FIELD

The present invention relates to improved laminar composite structuresand to a process for their manufacture. In particular the presentinvention relates to high strength light weight, rigid, compositematerials and to their manufacture. The invention further relates to theproduction of high strength, light weight articles from such composites.

BACKGROUND OF THE INVENTION

Steel is typically used in structures where high strength is required.However steel tends to be heavy and thus adds excess weight to thearticle. Although there are a few light weight materials which arestronger than steel they are extremely expensive.

SUMMARY OF THE INVENTION

Accordingly, there is provided a composite and a process of forming acomposite. The composite typically comprises a sandwich structurecomprising at least two surface layers attached to a central layer ofrigid epoxy foam wherein the layer of epoxy foam is at least 1.5 timesthe combined thickness of the two surface layers and the foam has adensity of from 0.2 to 1.5 gram/cc. In a preferred embodiment, thesurface layers are of metal foil such as aluminium or steel foil,plastic film or sheeting such as polypropylene or polyethylene film orpolyethylene terephthalate film. The surface layers can be porous,fibrous or both.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by the accompanying FIGS. 1 to 8 inwhich

FIG. 1 shows a sheet composite according to the present invention.

FIG. 2 shows a tube according to the present invention.

FIG. 3 shows an embodiment in which a metal insert has been positionedin the foamable material before foaming.

FIG. 4 is a photomicrograph of a cross section of one external surfaceof the composite produced according to the following Example.

FIG. 5 is a graph which compares the performance of the material of thepresent invention with that of a current high strength aluminium alloy.

FIG. 6 shows three tubes according to the present teachings.

FIG. 7 shows a tube being subjected to a bending test.

FIG. 8 shows a graph comparing the composite of the present teachingsversus an aluminium tube and a steel tube.

DETAILED DESCRIPTION OF THE INVENTION

We have found that the light weight, rigid composites of the presentinvention are particularly useful as materials in the transportationindustries such as in automotive, aircraft and shipping industries wherethey may be used to replace metal and glass reinforced plastic articlessuch as panels and reinforcements. In recent years there has been atrend to replace traditional steel components with lighter materials ofcomparable strength such as aluminium, fibre reinforced polymericmaterials, foam materials and composites particularly compositescontaining foamed layers. There is however a continuing need formaterials of increased strength and reduced weight.

The composites of the present invention have a wide variety of uses inadditional applications in which high strength combined with lightweight is required and in particular it can provide an inexpensive lightweight material with comparable or improved strength relative to steel.The composites are particularly useful in applications in which steelhas been used such as metal tubing, metal structures employed inconstruction and all forms of transportation. In addition, thestructures can be used to improve the strength of articles in whichlighter materials such as aluminium are used. Other uses include as rawmaterials in the production of sporting goods such as skis and in lineskates and in the production of furniture.

The strength required of a material will depend upon the use to which itis to be put. For example the important characteristics can be hightensile strength and high flexural modules as measured by ASTM D790/ISO178 norm or alternatively it can be resistance to impact, compressionstrength or torsional strength and in certain uses a combination ofthese properties may be required.

We have found that particularly desirable properties and in particularlythe combination of low weight and high strength and stiffness can beobtained from composites sandwich structures consisting of at least twosurface layers enclosing a layer of rigid epoxy foam.

Composite sandwich structures with a foam core are known and have beenproposed as materials having significant strength and stiffness togetherwith an advantage derived from weight considerations. For example theabstract of Japanese Patent publication JP 58049223 A2 disclosessandwich structures comprising epoxy foam sandwiched between two metalplates. Two articles by S. Venkatraman and Kishore, the first in theJournal of Reinforced Plastics and Composites Vol 16 No. 7/1997 and thesecond in the Journal of Reinforced Plastics and Composites Vol 17 No.8/1998 disclose composites comprising a thin layer of flexible foamsandwiched between two thick layers of glass-epoxy resin materials. Thefirst of these articles relates to impact studies on the glass/epoxylaminates and the second to Investigations on the role of foam layers inthe Failure of Glass-Epoxy composite subjected to repeated impacts. Inboth articles the layer of flexible epoxy foam is provided as aprefoamed flexible layer and is adhered to the glass/epoxy layer bymeans of an adhesive. The later article concludes that the way the sheetlayers of flexible foam are arranged with respect to the direction ofimpact influences the spread of the crack path on repeated impact.

U.S. Pat. No. 3,598,671 discloses a method of preparing foam plasticlaminated structures in which at least one component of a foam formingmaterial is applied as a coating onto the surface of one sheet of a basematerial. A further component of the foam forming material is applied asa coating onto the surface of a second sheet of the base material. Thesheets are then brought together so that the components of the foamforming materials are brought together so that they foam and produce afoamed core plastic laminated structure. Example 2 of U.S. Pat. No.3,598,671 produces such a laminate comprising a layer of epoxy foam ¼inch thick sandwiched between two layers of fiberglass eachapproximately ⅛ inch thick. Accordingly the combined thickness of thetwo layers of fiberglass is substantially the same as the thickness ofthe foam. The foam is also extremely heavy, having a density of 7.5 lbsper square foot. U.S. Pat. No. 3,598,671 does not therefore envisage thehigh strength light weight materials of the present invention.

Heat activatable foamable epoxy materials are known and are used in theproduction of structural reinforcement in automobiles. For instance thefoamable material may be applied to the surface of a metal or plasticcarrier to produce a component which is inserted into a part of thevehicle structure which requires reinforcement. The heat activatablefoamable epoxy material may be formulated so that it will foam under theconditions that prevail in the electrocoat (e-coat) process used toprovide an anticorrosion coating to the metal surfaces of the vehiclestructure or in any other painting operations. Such foamable epoxymaterials and their uses are described in U.S. Pat. Nos. 4,922,596;4,978,562; 5,124,186 and 5,884,960. We have now found that improvedlight weight high strength composite materials can be obtained fromfoamable epoxy materials of this type. U.S. patent application Ser. No.09/939,152 discloses structurally reinforced panels comprising a metalpanel, a woven roving and bonded to one side of a matrix material whichmay be an epoxy foam.

The present invention therefore provides a composite comprising asandwich structure comprising at least two surface layers attached to acentral layer of rigid epoxy foam wherein the layer of epoxy foam is atleast 1.5 times the combined thickness of the two surface layers and thefoam has a density of from 0.2 to 1.5 gram/cc preferably between 0.4 and1.5 gram/cc. In particular we prefer that the foam have a density of 0.3to 0.6 gram/cc.

In a further embodiment the invention provides a composite comprising atleast two surface layers each layer having a thickness of from 0.2 to 10millimetres and a core layer of a rigid epoxy foam having a thickness offrom 2 to 200 millimetres.

In a further embodiment of the present invention the composites of thepresent invention have a density in the range of from 0.1 to 2.0 gram/ccpreferably 0.1 to 1.0 gram/cc; the density depending on the nature ofthe material used for the surface layers. In yet a further embodiment acomposite of the present invention which is from 5 to 8 millimetersthick has a flexural modulus of from 100 mPa to 700 mPa preferably 200mPa more preferably 200 mPa to 700 mPa as measured by ASTM D 790/ISO 178norm.

The preferred surface layers of the composites of the present inventionmay be of any suitable material. Examples of suitable material includemetal, including metal foil such as aluminium or steel foil, plasticfilm or sheeting such as polypropylene or polyethylene film orpolyethylene terephthalate film. It is preferred however that thematerial be a fibrous material. It is particularly preferred that thesurface layers be porous so that the epoxy material can penetrate thepores in the surface layers so that the surface layers become embeddedin the epoxy foam. The invention is particularly useful in theproduction of light weight composite materials of hollow cross sectionsuch as tubing or box structures and here the surfaces may for examplebe concentric tubes or matching box sections of plastic or metal,aluminium being preferred. The surface layers may be the same ordifferent and in some embodiments the layers may be selected to providedesired properties for example in a composite hollow section such as atube the outer layer may be of a metal such as aluminium to provideshock resistance and the inner layer a tube such as carbon fibre toprovide strength.

Where fibrous material is used it may be of any suitable material andits selection will depend upon the use to which the composite materialis to be put. Examples of fibrous materials that may be used includewoven and non woven textile webs such as webs derived from polyester,polyamide, polyolefin, paper, carbon and Kevlar fibre. These webs may bewoven or obtained by non woven web manufacturing techniques such asneedle punching and point bonding. Metallic fibrous webs may also beused although we prefer to use glass fibre which may also be woven ornon woven. In particular we prefer to use glass fibre web having aweight of from 40 gram/sq metre to 400 gram/sq metre. Other preferredfibrous materials include carbon fibre and Kevlar. The surface layersmay themselves be produced by the laying up of two or more layers ofmaterial which may be the same or different.

The term embedded is used to describe a composite in which the surfacelayer, although at the surfaces of the composite is largely enveloped byepoxy material. This may be determined from electron micrographphotographs of a cross section of the material such as FIG. 4, whichshows a substantially continuous layer of epoxy material extending intoand at times through the surface layer. It will be appreciated that inorder to be embedded it is not essential that all of the surfacematerial be encased with the epoxy material. The epoxy material thatextends into and sometimes through the surface layer may be the same asthe epoxy foam although in a preferred embodiment of the presentinvention it is a separate unfoamed epoxy material that is compatiblewith the foamable epoxy material and forms a substantially continuousmatrix with the foamable material, taking into account the voids formeddue to the foaming of the epoxy material. The surface layer material istherefore embedded in this epoxy matrix.

The foam layer is of a rigid epoxy foam. Rigid meaning that it is hardto the touch and resistant to manually applied pressure. It is preferredthat the foam layer have a thickness of from 5 to 15 millimetres,preferably from 7 to 13 millimetres and most preferably from 8 to 13millimetres. In the production of the composite materials of the presentinvention it is preferred that the foamable material from which the foamis produced have a thickness in the unfoamed state of from 1 to 5millimetres, preferably 2 to 4 millimetres more preferably 2 to 3.5millimetres.

The present invention further provides a process for the production ofcomposite materials comprising providing a first layer, laying a layerof heat activatable foamable epoxy material thereon and providing asecond surface layer on the surface of the layer of heat activatablefoamable epoxy material remote from the first layer and heating toactivate the epoxy material to cause it to foam and cure and to therebyform a rigid foamed epoxy material bonded to the surface layers.

In one further embodiment the present invention provides another processfor the production of composite materials comprising spraying a foamableepoxy material between two surface layers and allowing the foamablematerial to expand and cure and bond to the surface layers.

In a preferred embodiment of the processes of the present invention thesurface layers are porous preferably of fibrous material. In a furtherpreferred embodiment the surface layers are coated and/or impregnatedwith an epoxy material prior to heating. Preferably when such an epoxymaterial is used it is compatible with the heat activatable foamableepoxy material, so as to form a substantially continuous matrixtherewith, as hereinbefore described. In this way when the surfacelayers are porous they can become embedded in the epoxy material.Preferably the epoxy material also cures under the same conditions asthe heat activatable foamable epoxy material cures. In this embodimentthe heating step of the process of the present invention will cure bothepoxy resins. Conveniently the epoxy material with which the porouslayers are coated and/or impregnated is the same epoxy material as isused as the basis for the heat activated foamable material although itneed not be foamable.

In another preferred embodiment the layers are matching hollow profilesand the foam is created between the profiles. For example the layers maybe concentric tubes or matching box sections. In this embodiment thestructures may be produced by providing the foamable material betweenthe two matching profiles which are preferably held apart to allowformation of the foam layer of desired thickness. For example thefoamable material may be extruded between two moving spaced apartconcentric tubes and subsequently foamed to produce the composite of theinvention.

The various embodiments of the present invention envisage in addition,sandwich composites containing four or more layers, their production andmaterials made therefrom. For example in addition to three layersandwich structures the composite may comprise five layers consisting oftwo outer layers such as fibrous, metallic or plastic layers and aninner layer of fibrous, metallic or plastic material with two layers ofepoxy foam or heat activatable epoxy foam forming material interposedbetween the layers. Composites containing a greater number of layers arealso envisaged providing, however, that the two surface layers of thecomposite are according to the present invention.

Foamable epoxy materials typically contain an epoxy resin, a blowingagent and a curing agent and frequently also contain a filler. Theblowing agent and the curing agent can be selected so that foaming andcuring (hardening) occur within the desired temperature range. Thematerials should therefore be chosen so that the temperature requiredfor foaming and curing does not damage the surface layers. The epoxyresin may be chosen according to the degree of stiffness that isrequired in the product. Amine curing agents are frequently used inwhich case curing temperatures of at least 100° C. are generallyrequired. It is preferred that the blowing agent and curing agent bechosen so that foaming starts at a temperature slightly below the curingtemperature. The foamable epoxy resin may be applied as a liquidtypically through use of a solvent such as an alcohol. In thisembodiment the epoxy resin may be sprayed or painted onto one or both ofthe surface layers. The solvent may then be removed by evaporation toprovide a continuous or discontinuous layer of foamable epoxy material.Alternatively the foamable epoxy material may be extruded and cut intopieces for use in the invention or alternatively extruded onto one ofthe surface layers conveniently between the two surface layers.

The heat activated epoxy foam forming material should be chosenaccording to the application to which the composite is to be put.However the heat-activated epoxy-based resin should have foamablecharacteristics upon activation through the use of heat whereby itexpands, cross-links to produce hard, rigid foam and cures to bond tothe surface layers. An example of a preferred formulation is anepoxy-based material that may include polymer modifiers such as anethylene copolymer or terpolymer that is commercially available from L &L Products, Inc of Romeo, Mich., under the designations L-5204, L-5206,L-5207, L-5208, L-5209, L-5214 and L-5222 and from Core Products as core5207,5214, 5234 and 5231. These products may also include fillers suchas glass microspheres, calcium carbonate and talc which can reduce thedensity of the foam. One advantage of these preferred heat activatedfoamable materials is that they can be processed in several ways toproduce the heat activatable foamable layer of the present invention.The layer of foamable activatable epoxy material may be continuous ordiscontinuous. Possible techniques for the provision of the layer offoamable activatable material include the provision of sheet material,injection molding, blow molding, thermoforming, direct deposition ofpalletized materials, extrusion or extrusion with a mini-applicatorextruder. The preferred epoxy materials are flexible prior to activationand this flexibility enables the creation of designs that allow theproduction of complex shapes and which exceed the design flexibilitycapability of most prior art high strength materials.

The heat activatable foamable epoxy resin whether it be as strips orspots is thermally expandable. That is, upon the application of heatthey will expand, typically by a foaming reaction and preferably to atleast 130%, more preferably at least 150%, the volume of the unexpandedstate, but more preferably to at least twice the volume of the expandedstate. The material also cures to provide a rigid epoxy foam bonded tothe surface layers. The foamable material is preferably not tacky to thetouch at room temperature and it is such that it will soften and thenexpand due to the activation of the blowing agent, the epoxy will thenbegin to cure and develop adhesive properties so that it will bond tothe surface layers and finally the curing will be completed to hardenthe foamed epoxy resin. The resulting product being a sandwich structurecomprising the surface layers bonded to the hard, rigid epoxy foam.

Epoxy resin preferably forms about 5% to about 75% by weight and morepreferably from about 15% to 65% by weight of the activatable foamableepoxy material composition. Filler preferably forms from about 0% toabout 70% by weight and more preferably from about 20% to about 50% byweight of the composition. A blowing agent preferably forms from about0.5% to about 10% by weight and more preferably from about 0.2% to 5% byweight of the composition. A curing agent preferably forms from about 0%to about 10% by weight and more preferably from about 0.5% to 5% byweight of the composition. An accelerator preferably forms from about 0%to about 10% by weight and more preferably from about 0.3% to 5% byweight of the composition. A preferred formulation is set out in thefollowing table.

Ingredient % by weight Epoxy Resin 15% to 65% Ethylene Copolymer  0% to20% Blowing Agent 0.2% to 5%   Curing Agent 0.5% to 5   Accelerator 0.3%to 5%   Filler 20% to 50%

In the preferred embodiment of the process of the present invention oneor more of the surface layers is coated and/or impregnated with an epoxymaterial prior to heating to foam and cure the heat activatable epoxymaterial. In this embodiment it is preferred that the epoxy materialused to coat and/or impregnate the surface layer cure under the sameconditions as those under which the heat activatable foamable materialcures. In a particularly preferred embodiment the epoxy material is thesame as the epoxy material upon which the heat activated foamablematerial is based. This embodiment is particularly preferred when thesurface layers are porous so that they may be impregnated with epoxymaterial. Alternatively the impregnation may be accomplished through theapplication of the foamable epoxy material in liquid form.

Epoxy resins that are preferably used in the foamable material have anepoxy equivalents value of about 200 to 5000, more preferably 300 to3000, since these resins have suitable curing reactivity and meltingpoints. Therefore, foams having satisfactory rigidity can be prepared.More preferably, the epoxy resin contains about 500 to 2500 and mostpreferably contains about 500 to 1500 epoxy equivalents.

Epoxy resins having suitable epoxy equivalents are not restricted to asingle type of epoxy resin. Rather, combinations of epoxy resins may beused. Representative epoxy resins include, but are not limited to,bisphenol A, bisphenol F, brominated bisphenol A, hydrogenated bisphenolA, bisphenol S, bisphenol AF, biphenyl, naphthalene, fluorine, phenolnovolac, ortho-cresone novolac, DPP novolac, trifunctional,tris-hydroxyphenylmethane, tetraphenolethane and other glycidyl ethertypes are preferred.

The curing agent is material that is capable of curing epoxy resins.Preferably, the curing agent can co-exist at room temperature or usualstorage temperatures with the epoxy resin in unexpanded form withoutreacting with the epoxy resin, while at the same time, maintaining itscuring reactivity. The curing agent preferably cures the epoxy resin ata temperature above the melting point of the epoxy resin and accordinglythe curing reactivity of the curing agent should not be diminished whenthe foamable epoxy resin formulation is compounded and/or extruded.

Preferably, the curing agent retains sufficient reactivity during theproduction and storage of the foamable material in order to cure theepoxy resin when desired. Therefore, while some limited curing of theepoxy resin may occur during the production and storage of the foamprecursor material, such curing should not substantially affect thecuring reactivity of the curing agent. Accordingly, the curing agentalso preferably has low reactivity with the epoxy resin when stored atroom or usual storage temperatures in order to allow the foamablematerial to be stored for a long-term. Preferably, a curing agent isutilised that initiates curing in the temperature range of about 100° C.to 200°C. More preferably, the curing temperature is from about 130°C.to 180°C.

Preferred curing agents include polyaddition type, catalyst type andcondensation type curing agents. The polyaddition type curing agentsinclude, but are not limited to, polyamine-based dicyandiamide and theacid anhydride-based methyl nadic acid anhydride. The catalyst typecuring agents include, but are not limited to, Imidazole-based2-methylimidazole, 2-ethyl 4-methylimidazole and 2-heptadecyl Imidazole,Lewis acid-based monoethylamine boron trifluoride, piperazine borontrifluoride and other related compounds.

The amount of curing agent used in the foam precursor materials willdiffer depending on the epoxy equivalents of the epoxy resin and will besufficient to provide the rigid foam structure. Generally, an amount ofcuring agent is utilized that will effectively cure the epoxy resin.Preferable amounts are 1 to 25 parts by weight to 100 parts by weight ofthe epoxy resin and more preferably 1 to 10 parts by weight.

The foaming agent decomposes and expands at a temperature that is higherthan the melting point of the epoxy resin that is used and does notdecompose during the compounding and production of the foamable materialsuch as during compounding and/or extrusion.

A foaming agent is selected that decomposes, and thus expands the epoxyresin, in a prescribed heating temperature range, depending on thecuring agent used. The temperature range in which the curing agent cancure the epoxy resin should preferably overlap the temperature range inwhich the foaming agent decomposes and expands. Specifically, thefoaming (decomposition) temperature is preferably about 100° C. orhigher, and more preferably 120°C. or higher.

Preferably, organic decomposing-type foaming agents are utilised. Forexample, azodicarbonamide, azobisformamide, azobisisobutyronitrile,barium azodicarboxylate, N,N¹-dinitrosopentamethylene tetramine, N,N¹-dinitroso-N,N1-dimethylteraphthalamide, para-toluensulfonylhydrazide, benzenesulfonyl hydrazide, 4,4¹-oxybenzenesulfonyl hydrazideand other related compounds may be used. Any one or a combination of twoor more of these foaming agents may be used. Azodicarbonamide isparticularly preferred.

The foaming agent is preferably added in an amount to provide a foamingratio of about 30% to 100%, preferably 50% to 400%, and more preferablyabout 100% to 300%. That is, the amount of foaming agent utilised willproduce a foam material having a volume 1.3 to 10 times larger than thevolume of the foam precursor material, preferably 1.5 to 5 times largerand more preferably about 2 to 4 times larger. While specific amountswill depend upon the particular epoxy resin that is selected, thefoaming agent may be added at about 0.5 to 15 parts by weight to 100parts by weight of the epoxy resin and more preferably 0.5 to 10 partsby weight.

Various other additives may be added to the foamable composition, suchas other resin components including for example, thermosetting resinsand/or thermoplastic resins, inorganic additives including, for example,calcium carbonate, talc or mica, reactive dilutive compositions, curingaccelerators, foaming aids, flame retardants, colouring agents andreinforcing materials (in powder form, chip form, fibre form bead form,etc) including, for example, glass, metal, ceramic or similar materials.

A thermosetting resin, other than an epoxy resin, may be added,including for example, polyester resins, melamine resins, urea resinsand phenol resins. If such thermosetting resins are used, the amount ofthermosetting resin can be appropriately determined based upon on thecuring agent. If a thermoplastic resin is added, the resulting foam isgenerally more resilient than when a thermosetting resin alone is used.Thus, by changing the amount of thermoplastic resins and thermosettingresins that are included in the foam precursor material, foam materialshaving different qualities may be produced. For example, adding suchadditives can increase the toughness of the foam material.

If a thermoplastic resin is added as an additive, the resin component ispreferably polyethylene, polyvinyl acetate or a copolymer of ethyleneand an alkyl acrylate. The copolymers of ethylene and alkyl acrylatescan include ethylene-methyl acrylate polymer, ethylene-ethyl acrylatepolymer, ethylene-butyl acrylate polymer etc, and preferred copolymersare of ethylene and alkyl acrylates with about 1 to 4 carbon atoms inthe alkyl group. The thermoplastic resin may be a single compound orcomposition of matter or a combination of two or more compounds orcompositions of matter.

If a thermoplastic resin is added, a polymerizable monomer may also beadded. Suitable polymerizable monomers include triallyl cyanurate,triallyl isocryanurate, trimethylolpropane trimethacrylate and similarcompounds.

By adding a reinforcing material (in powder form, chip form, fibre form,bead form etc) such as glass, metal or ceramic, the rigidity of theresulting foam material can be increased. Specifically, if afibre-formed material is added, the resiliency of the resulting foammaterial can be increased. The amount of such reinforcing materials tobe added is preferably 1 to 300 parts by weight to 100 parts by weightof the resin component and more preferably 1 to 100 parts by weight.

In the embodiment of the invention where the surface layers are fibrouslayers, which are preferably carbon fibre or glass fibre layers aspreviously described and are coated and/or impregnated with an epoxymaterial comprising epoxy resin preferably from about 5% to about 75% byweight and more preferably from about 15% to 65% by weight of thecomposition. Filler preferably forms from about 0% to about 70% byweight and more preferably from about 20% to about 50% by weight of thecomposition. Curing agent preferably forms from about 0% to about 10% byweight and more preferably from about 0.5% to 5% by weight of thecomposition. Accelerator preferably forms from about 0% to about 10% byweight and more preferably from about 0.3% to 5% by weight of thecomposition. A preferred coating formulation is set out in the followingtable.

Ingredient % by weight Epoxy Resin 15% to 65% Ethylene Copolymer  0% to20% Curing Agent 0.5% to 5   Accelerator 0.3% to 5%   Filler 20% to 50%

The composites of the present invention may be of any required shape andmay conveniently be formed in a mould designed to provide the requiredshape. In certain embodiments the surface layers and the heatactivatable foamable material (prior to foaming) are generally flexiblematerials. Accordingly the composites may be produced by laying down thefirst surface layer in the mold, optionally coating and/or impregnatingsaid first layer with an epoxy material, then providing a layer of theheat activatable foamable epoxy material against the first surfacelayer, providing a second surface layer against the layer of heatactivatable material. This second layer may optionally be precoatedand/or impregnated with an epoxy material or, optionally, coated and/orimpregnated after it is provided against the layer of heat activatablematerial. The mold may then be closed and heated to the temperaturerequired to cause the heat activatable foamable epoxy material to foamand cure and to cause any epoxy material used to coat and/or impregnatethe surface layers to cure. Where composites having more than threelayers are to be produced additional layers may be provided prior toclosing the mould and heating to cause foaming and curing. Theadditional layers may be of any suitable material such as continuoussheet or fibrous layers.

Alternatively the surface layers may be rigid such as matching boxstructures or concentric tubes and in this embodiment the foamable epoxymaterial may be provided between the matching structures or tubes whichare held apart by suitable means to allow the desired expansion of thefoamable material. In a preferred embodiment the box structures or tubesare metallic.

The temperatures that should be used for the heating to foam and curethe epoxy materials will depend upon the choice of blowing agent andcuring agent however, we have found that temperatures in the region of100° C. to 240° C. are particularly useful although epoxy resin systemsthat foam upon mixing the components at lower temperatures are known.Alternatively one can use a system which foams at lower temperatures,such as those in which two or more components are provided and mixed atambient temperature. A benefit of the process of the present inventionis that external pressure may not be required during moulding in orderto achieve a desired surface finish in that the foaming of the heatactivatable foamable epoxy resin can itself produced sufficient internalpressure.

Following the molding process the mold may be opened and the desiredcomposite obtained. If necessary the mold surface may be provided withrelease lining material to ensure the composite does not adhere to themold.

We have found that composites of the present invention have highflexural strength combined with low weight. Typically a composite havinga thickness of from 5 millimetres to 8 millimetres has a flexuralstrength of from 100 mPa to 700 mPa typically 300 mPa to 700 mPa at adensity of from 0.1 to 1.0 grams/cc which compares favourably withcurrent light weight aluminium, based materials of similar weight perunit area which have a flexural modulus of about 10% that of thecomposite of the present invention products of comparable weight perunit area. We have also found that the composites of the presentinvention can sustain considerably greater maximum load than currentsteel box structures or tubular structures; the composites are alsolight weight particularly if they are aluminium based. They thereforeoffer a strong system with considerable weight saving.

In addition we have found that a composite of the present invention hasa considerably greater elasticity range under load than comparable steeland aluminium based materials. The improved elasticity beingdemonstrated by an extension of the elastic area under increasing loadin a standard three point bend test.

The composites of the present invention find a wide range of uses wherehigh strength and light weight are required. For example they may beused in the construction industry, in transportation industriesincluding the automobile, aircraft, aerospace and shipping vesselindustries. The composite may be used in applications where box sectionsor tubes are used to provide strength and/or reinforcement. For examplethey have found use in aircraft and in particular light weight unmannedsurveillance aircraft where they can provide a light weight tubularstructure and can also provide strong, light weight panels forsupporting equipment. Other applications include reinforcement invehicles such as door reinforcement against vehicle front and side crashwhere the composites may be used in applications which currently employsteel structures to provide increased strength at reduced weight. It isclear that similar benefits may be accomplished in a multitude ofapplications where the combination of strength and light weight arerequired. Furthermore they may be used in the production of sportinggoods such as skis, roller skates, roller blades and the like.

The composites may also be used as components in buildings, vehicles,sporting goods and furniture.

If the composites are to be attached to other components within thefinished article attachment means may be provided within the compositeto enable assembly. In this embodiment of the invention the attachmentmay be located within the activatable foamable material prior to heatingto foam and cure. In this way the foamable material can expand aroundthe attachment to hold it firmly in place as the epoxy material cures.Typical attachment means comprise clips, studs, bolts and the like whichmay be of any material providing they retain their strength under theconditions used for activation and foaming.

The present invention is further illustrated by reference to thefollowing Examples.

Example 1

An extruded foamable epoxy material available from Core Products asCORE-5234 is placed between two layers of a woven carbon fibre matt ofweight 245 grams/m², impregnated with the same epoxy resins as was usedfor the foamable materials. The three layers are then placed in a mouldand cured at 175° C. for 45 minutes and at normal pressure to form acomposite 5.4 millimetres thick.

Example 2

The process of example 1 is repeated, except the fibre roving is notimpregnated with a separate epoxy resin but with the foamable materialitself which has been modified to be liquid with a methanol solvent.

The difference is that this composite sandwich (embedded fibrerowing+foamable material+embedded fibre) is now dry to touch and can beeasily handled.

FIG. 5 compares the flexural modulus according to ASTM D790/ISO 178 of asample of the composite produced in Example 1 with a sample oftraditional high strength aluminium of thickness 1.25 mm and of similarweight per unit area. The composite of the present invention had adensity of 0.5 grams/cc whereas the density of the aluminium layer is2.7 grams/cc. The composite of the present invention weighed 6.955grams/sq metre whereas the aluminium sample weighed 7.13 grams/sq metre.FIG. 5 shows that the composite of the invention had a flexural modulusof 360 mPa as compared with a modulus of 31.5 mPa for the aluminiumsample. FIG. 5 also shows that the maximum load that can be sustained bythe composite of the invention is 590 Newtons whereas that for thealuminium sample is 100 Newtons.

The invention is further illustrated by the accompanying FIG. 6 whichshows three tubes.

-   -   a) A steel tube of 35 millimetres external diameter and of steel        thickness 1.5 millimetres of weight 1133 grams per metre and        density 7.2 grams/cc.    -   b) An aluminium tube of 35 millimetres external diameter and of        aluminium thickness 1.5 millimetres of weight 425 grams per        metre and density 2.7 grams/cc.    -   c) A composite tube according to the present invention of weight        837 grams per metre and density 1.6 grams/cc consisting of an        outer aluminium tube of 35 millimetres external diameter and of        aluminium thickness 1.5 millimetres, an inner aluminium tube of        25 millimetres external diameter and of aluminium thickness 1        millimetre. A layer of rigid epoxy foam of thickness 4.25        millimetres is contained between the concentric aluminium tubes,        the foam density is 0.66 grams/cc.

The product of the invention was prepared by extruding a foamable epoxyresin composition from a formulation available from Core Products asCore 5207 to provide a strip of foamable material 3 millimetres thick.The strip was then cut and applied manually around the internalaluminium tube. The outer tube was then provided around the foam stripwith plugs provided at both ends to define and maintain a gap betweenthe foamable material and the inner surface of the outer tube. Thestructure was then heated at 170° C. for 30 minutes to cause thefoamable material to expand and adhere to both metal tubes to producethe structure shown in (c) of FIG. 6.

The tubes were subjected to a bending test using a three point bendingtest machine as shown in FIG. 7 and the results of the tests are shownin FIG. 8. The applied speed for the bending test was 20 mm/min.

FIG. 8 shows that the composite of the invention had a greater elasticrange (to 5000N) as compared with the steel tube (to about 3250N) andthe aluminium tube (to about 1250N). Furthermore the strength of thecomposite was considerably greater than both the steel and the aluminiumtubes with a resistance to about 7250N for the composition as comparedwith about 4750N for the steel tube and about 3250N for the aluminiumtube. The results clearly show the improvement in strength obtained withthe composite of the invention and with a significant reduction inweight.

1. A composite comprising a sandwich structure comprising at least twocarbon fiber surface layers attached to a central layer of rigidtrimethacrylate-based foam wherein the layer of foam is at least 1.5times the combined thickness of the two surface layers and the compositehas a density of from 0.1 to 2.0 gram/cc.
 2. A composite according toclaim 1 in which the foam has a density of between 0.2 and 1.5 gram/cc.3. A composite according to claim 1 in which the foam has a density of0.3 to 0.6 gram/cc.
 4. A composite according to claim 1, wherein the atleast two surface layers each have a thickness of from 0.2 to 10millimeters and the layer of foam having a thickness of from 2 to 200millimeters.
 5. A composite according to claim 1, having a flexuralmodulus as measured by ASTM D790/ISO 178 from 200 mPa to 700 mPa.
 6. Acomposite according to claim 1, in which the composite includes insertsor layers selected from metal foil such as aluminum or steel foil,plastic film or sheeting such as polypropylene or polyethylene film orpolyethylene terephthalate film.
 7. A composite according to claim 1, inwhich the surface layers are porous.
 8. A composite according to claim 5in which the surface layers are fibrous and porous. 9-10. (canceled) 11.A composite according to claim 1, in which the surface layers arematching internal and external structures.
 12. A composite according toclaim 1, in which the composites form hollow box sections.
 13. Acomposite according to claim 1, in which the composites form tubes.14-31. (canceled)
 32. A composite according to claim 4, in which thefoam has a density of between 0.2 and 1.5 gram/cc.
 33. A compositeaccording to claim 6, in which the foam has a density of between 0.2 and1.5 gram/cc.
 34. A composite according to claim 11, in which the foamhas a density of between 0.2 and 1.5 gram/cc.
 35. A composite accordingto claim 4, wherein the at least two surface layers each have athickness of from 0.2 to 10 millimeters and the layer of foam having athickness of from 2 to 200 millimeters.
 36. A composite according toclaim 6, wherein the at least two surface layers each have a thicknessof from 0.2 to 10 millimeters and the layer of foam having a thicknessof from 2 to 200 millimeters.
 37. A composite according to claim 11,wherein the at least two surface layers each have a thickness of from0.2 to 10 millimeters and the layer of foam having a thickness of from 2to 200 millimeters.
 38. A composite according to claim 4, in which thecomposite includes inserts or layers selected from metal foil such asaluminum or steel foil, plastic film or sheeting such as polypropyleneor polyethylene film or polyethylene terephthalate film.
 39. A compositeaccording to claim 11, in which the composite includes inserts or layersselected from metal foil such as aluminum or steel foil, plastic film orsheeting such as polypropylene or polyethylene film or polyethyleneterephthalate film.
 40. A composite according to claim 4, in which thecomposites form tubes.